Under the conditions of thermo-printing or transfer printing, sublimable dyes are transferred through heating from an auxiliary image support to the substrate to be printed, and are then fixed on the substrate, e.g., a textile fabric made of synthetic resin fibers, the fixing being effected through diffusion or dissolving into the fibrous material, the dyes thereby producing the desired colored images on the textile. As textile materials suitable for transfer printing, there are commonly used synthetic fibrous materials such as polyamides, polyesters, polyacrylonitriles and also cellulose fibers finished with synthetic resin. As the auxiliary image support, one uses generally in this connection paper sheets or metal foil, such as aluminum foil, on which the printing inks are applied by means of intaglio printing. As printing-ink binders suitable for this kind of printing, one uses in particular the cellulose esters, e.g., ethyl cellulose, in which connection one must use organic solvents.
However, it is also known to prepare such thermo-printing transfer papers through offset or letter printing, which introduces known advantages in relation to intaglio printing. Smaller editions can thus be produced without difficulty by means of this printing technique. Besides, offset or letter printing exhibits the advantage of omitting volatile organic solvents in the printing pastes, which leads to an acceleration of the printing operation, and, beyond that, introduces economic advantages.
In order to allow faster processing of the oily moist transfer-printing papers initially obtained in connection with offset printing, the German disclosed specification DT-OS 24 08 900 indicates the addition of gelling agents, such as aluminum chelate, in the amounts of up to 2% by weight to the printing-ink masses. This indeed produces a print which solidifies very rapidly on the surface; however, it has been shown quite generally that difficulties in regard to the reproducibility of color tones and color intensity of the thermo-transfer prints occur in connection with color images printed in accordance with the offset procedure. This is based on the fact that standard preparations are employed as binders for the manufacture of the printing inks for the thermo-printing papers through offset printing, such preparations being obtained on the basis of drying oils, such as linseed oil and mineral oils, unsaturated alkyd resins, hard resins or the like. As is well known, such printing inks dry up only very slowly through oxidation and, under the effect of air oxygen, attain a more or less definitive final state of crosslinking and, therewith, drying and hardening or curing only after a number of days.
This fact plays only a subordinate role in ordinary offset printing and is compensated and masked by other advantages offered by offset printing. However, in the preparation of thermo-transfer printing papers, hereafter referred to as "heat-transfers", it has turned out that oxidative curing is of influence because it was shown that the dyes used for the thermo-transfer printing, e.g., the readily sublimable anthraquinone or diazo dyes, are also oxidized in part during the crosslinking and hardening by oxidation of the binders used in the offset printing paste. Thus, after only a short storage period, this leads to color tone changes, e.g., fading or loss of brilliance, etc., when the dyes are transferred to the textile. Both in the case where such sublimable dyes are used alone and, especially, also in the case of dye mixtures that are required for obtaining the desired richness of shade, distinct differences in regard to tones and effect of the color images obtained through the transfer printing of textiles result within the drying time and then also during storage.
For the manufacturer of heat-transfers, e.g., the printer, this makes it very difficult or even impossible to exercise a precise control of his dye composition and color in the printing ink in regard to the result on the textile to be later obtained through transfer printing. While it is true that the oxidizing hardening of the offset printing inks can be accelerated through catalysts or by heating, catalysts also promote the undesirable oxidation of the dyes, and heating is conceivable only with the use of dyes possessing a high temperature of sublimation. These methods thus cannot be used practically in the manufacture of heat-transfers, especially since one must here use dyes that should sublime on heating as quickly as possible and as completely as possible.